Powdered coal furnace



Ma-ICh 23, 1937r W, H F|TCH POWDERED COAL FURNACE Filed June 12, 1935 2 Sheets-Sheet l ATTORNEY.

Patented Mar. 23, 1937 UNITED STATES PATENT `GFFICE Application June 12,

4 Claims.

This invention relates to a furnace arrangement in which powdered coal is burned in an economical manner as a consequence of the recuperator and pulverizer arrangements.

5 The invention relates more particularly to a metallurgical furnace in which preheated air is used to facilitate the complete combustion of the fuel and is `also used to dry the coal in the pulverzer. One of the objects of my invention is to provide a recuperator which is easily attached to a powdered coal furnace that has been previously operated without a recuperator. Some special features of the recuperator itself will be disclosed in detail below.

Objections have been raised to the use of pul- Verized coal as fuel in metallurgical melting furnaces in 'cases where `combustion 'of the fuel is not completed before it comes into direct contact with the metal. If the 'condition of the coal is such that complete combustion is not obtained,

the uncombined oxygen will 'attack the hot metal and voxidire more or less of it `depending on the character of the metal andother conditions. Incomplete combustion -of carbon and sulphur cause admixture of these materials with the metal. In the case of iron and .steel such admixture tends to `change greatly the physical and chemical characteristics of the metal, as for example its hardness, malleabil'ity, magnetic susceptibility and its 'resistance to corrosion.

AA great many of the difficulties which are met with in powdered coal-fired furnaces are due to moisture in the coal. The `presence of moisture in 'coal causes, for example, irregularity in the rate of feeding 'of coal to the burners and also causes a reduction of flame temperature due to the high 'heat of vaporization of water and due to the displacement of oxygen by water Vapor in 40 the combustion chamber. The amount of moisture in coal which is stored outside varies with the Weather conditions. The use of air at ordinary atmospheric temperature and of average relative humidity with Wet `coal in Vthe pulverizer results in `slow land incomplete pulVeri-zation and leads to incomplete vcombustion of the coal in Vthe combustion chamber. -By the 'use of preheated air in a'inanne'r similarto lthat employed `by the applil cant in his improved 'furnace the average fuel consumption in a particular furnace has been decreased 22 per cent. This decrease in the amount of coal used also improves the quality of the metallic product.

` In the 'case of one way metallurgical furnaces 1933, Serial No. 675,402`

using powdered coal, the use of recuperators has generally been considered impractical on account of the intermittent nature of their operation. Many furnaces make only oneheat a day thus limiting the time of operation to 8 hours per day. With recuperator tiles made of fire-clay too large a proportion of the heating time is taken up Iin heating the tiles to make them efficient in heat transmission, especially in the case of intermittent operation. The thermal conductivity of revclay is only about one-ninth of that of silicon carbide, thus making the use of fire-clay impracticable for most recuperator heat transmission elements.

The various improvements which I have made in my furnace arrangement and in the recuperator itself are illustrated by the accompanying drawings in which:

Figure 1 is an elevation partly diagrammatic and partly in section showing the general arrangement ofmy improved furnace, the section being taken 'on the line I-I of Figure 2 and a portion 'of .the installation under the floor line being indicated at the left of the view;

Figure 2 is a plan View' of the improved 'furnace, partly in section, the section being taken on the line II-II of Figure 3;

Figure 3 is an enlarged sectional elevation of the recuperator chamber, the section being taken on the line III--III of Figure r2;

Figure 4 is an elevation viewed in the same direction as Figure 3 and showing the end of the recuperator which is lmost remote from the furnace; and

Figure 5 is a `fragmentary enlarged horizontal section of a portion of the recuperator wall that supports the air inlet ends of the recuperator tubes, the section being taken through the upper portion of the recuperator tubes and illustrating the calking of the Vjoints in the wall.

Referring to the drawings in detail, the furnace 2 which is illustrated by way of example is a one-way melting furnace. It is fired with powdered coal fuel which is distributed along with preheated air to the burner inlets 3, 3. The coal is reduced to its powdered state in the pulverizer 4. The waste gases pass out of the furnace into a stack 5. The waste gases pass from the lower section of the 'stack into the recuperator 1 at the inlet 8, thence up around and between the recupera'tor tubes I2, and then (in a much cooled coni dition) into the upper stack at the recuperator outlet 9. Doors 30 `are indicated in the sides of the furnace. Peep holes 3l are indicated for ob-u direction through the recuperator tubes I2 into a conduit I3. These recuperator tubes are made vof a very highly refractory material having a high thermal conductivity and a low coeicient of thermal expansion, such as silicon carbide. The

f conduit I3 is insulated and is conveniently taken l under the floor to a point adjacent the burners `themselves.

3, 3', the heated air being moved by means of the suction pump I4. The latter is driven by means of a motor I5. A by-pass pipe I6 leaves the duct I3 at a point near the pulverizer where it dries the coal and renders the latter more suitable for uniform pulverization and burning. A return air pipe 20 leaves the lower part of the pulversizer and re-enters the duct I3 at an angle as shown in Fig. 2. The powdered coal is mixed with the main stream of preheated air in the outlet of the pump I4 and passes through the pipes I9 into the burners 3, 3.

The recuperator 'I is mounted well above the shop floor on posts I'I in such a manner that the installation of the recuperator does not take up appreciable floor space. During this installation openings are made in the stack at 8 and 9 above the furnace, as shown in Fig. 1, and a baille 6 is placed in position in the stack at a height approximately equal to the mean height of the recuperator chamber. The recuperator tubes I2 are placed horizontally as shown in Figures 1, 2 and 3 and are made of a refractory material having a thermal conductivity greater than 0.006 calorie /cm3/sec/C., as for example bonded silicon carbide.

The recuperator tubes extend across the recuperator chamber 'I and are supported around their terminal portions by means of fire-clay blocks which are shaped to hold the cylindrical terminal surfaces of the tubes with small clearance. A plastic cement is used to ll the joints between the recuperator tubes and the supporting blocks and also between the refractory blocks The combustion gases pass around the exterior surfaces of the intermediate portions of the recuperator tubes which are exposed to the upwardly flowing combustion gases. Heat is thus transmitted from the combustion gases to the air which ows through the recuperator tubes.

The elevated position of the recuperator chamber permits much precipitation of slag from the waste gases before the latter reach the recuperator tubes. Refractory blocks 2l are used to form the walls in which the tube ends rest, and are designed to provide supporting surfaces which are coaxial with the axes of the corresponding recuperator tubes. By using suitable cementing material between the blocks and between blocks and tubes, the entire chamber can be constructed yto resist leakage at the pressures employed in the operation of my improved furnace. The

edges of the blocks are undercut as indicated `at 22 in Fig. 5 so that expansion of the blocks tends to compress the plastic cement in the undercut portions. On account of the alternate expansion and contraction of the tubesand walls of the recuperator chamber leaks will eventually A develop in the cemented joints. After a series of furnace runs the joints are recalked from the side of the air intake and similarly repaired in the case of the opposite wall, access, being obtained in the latter case by means of a manhole in the conduit I3.

The two walls which support the recuperator tubes may be called the tube terminal walls, and the other two walls may be called the side walls. It has been common practice in building such recuperator chambers to join the tube terminal walls to the side walls by forming recesses in the latter to receive the tube terminal walls. With this construction it is difficult to prevent leakage between the waste gas flue and the usual air chamber which provides air for the recuperator tubes. In the applicants construction the recuperator tubes are connected directly to the atmosphere, thus eliminating the usual air chamber. With a satisfactory seal between the recuperator tubes and the surrounding walls, there is therefore little danger of contamination of the air which is being preheated in its passage through the recuperator chamber.

The recuperator tubes are provided with corebusters 23 which are indicated in Fig. 5. These corebusters are solid rods which are supported on short legs in such a position that the ow of air is confined to a thin layer whichvis n good conducting contact with the interior surfaces of the recuperator tubes. With this arrangement the velocity of the air to be preheated is increased over that which would obtain in similar' tubes having no corebusters and more intimate con tact is obtained between the air to be preheate and the recuperator tubes.

My recuperator is adapted to be attached to existing furnaces with a minimum of inconvenience. The construction of the recuperator walls which support the tubes is facilitated by the use of re-clay tile of the type shown at 2I in Figure 5. These blocks are set up with a thin layer of refractory cement in the joints indicated by the reference character 24 in Fig. 5. Such refractory cement may be made, for example, from finely divided calcined clay grog and sodium silicate. When the fire-clay blocks are set up in the manner indicated in Figure 5, there are cylindrical openings through the walls of a diameter slightly greater than the diameters of the recuperator tubes. On the outside faces of the wall section shown in Fig. 5 the edges of the blocks are undercut as indicated at 25 and 26 to form spaces which may be used for calking the joints between the blocks and the joints between the blocks and tubes. A cross-section of one of the openings 25 or 26 in the plane of Fig. 5 is trapezoidal, the base of the trapezoid being in the interior of the wall. the tubes in place the terminal portions are given a thin, even coating of the refractory cement described above and are placed with their terminal portions in the cylindrical openings of the tube terminal walls. 'Ihe openings 26 are then filled with a plastic refractory cement of the type known to the trade as Stic-Tite. The corresponding openings 25 between the adjacent blocks are lled with a similar plastic cement. tube walls is rising, the thermal expansion of the blocks and tubes tends to force Athe cement in ithe spaces 25 and 26 toward the narrow openings the rate of combustion and promotes complete` In mounting When the temperature of the terminal? ness of combustion. The danger of contamination of the metal by carbon is consequently decreased and the quality of the metal obtained is improved. My improved furnace and recuperator arrangement may also be used with gaseous or liquid fuels; but in view of the much higher cost of these fuels the use of powdered coal is desirable provided contamination of the metal with carbon and other impurities can be reduced below the permissible carbon limit in the manner which has been found to be reliably possible with my invention.

The type of furnace shown in the drawings is merely illustrative and my invention may be used with many types of powdered coal furnaces. The particular type of recuperator chamber which I have disclosed may be used with gas or oil-fired furnaces, but the by-pass arrangement by which a portion of the preheated air is shunted through a pulverizer is particularly adapted to powdered coal furnaces. The condition of the coal is thus improved both mechanically and thermally for conveyance to the burners.

I claim:

1. A fuel economizer for powdered coal furnaces comprising a combustion chamber, a stack that is closely connected to the outlet of the combustion chamber, a recuperator chamber built on the side of the stack and connected thereto by means of an opening from the stack situated above the combustion chamber and passing into the lower part of the recuperator chamber and also by means of an opening from the upper part of the recuperator chamber into the stack, a baille placed in the stack at a height intermediate of the heights of said lower opening and said upper opening, two opposite walls of the recuperator chamber being provided with openings, a plurality of non-metallic recuperator tubes communieating at their ends with said openings, an insulated conduit communicating with the openings in one of said walls, said recuperator tubes being composed of a material havinga thermal conductivity substantially greater than that of fire-clay, a fan having its intake communicating with said insulated conduit, powdered coal burners connected to the outlet of said fan and to said combustion chamber, a coal pulverizer adjacent said burners and communicating with said fan, and a short by-pass connection from said insulated conduit to said pulverizer for passing a portion of the preheated air into the pulverizer to improve the drying and pulverization of the coal prior to its injection into the burners.

2. A fuel economizer for powdered coal furnaces comprising a one-way furnace, a stack that is closely connected to the burned gas outlet of said furnace, a recuperator chamber built on to the side of said stack and connected thereto by means of an opening from the stack into the lower portion of the recuperator chamber at a height above the top of the furnace and also by means of an opening from the upper part of the recuperator chamber into the stack, a baille placed in the stack at a height intermediate of the heights of said lower opening and said upper opening, said recuperator chamber being elongated in a direction away from said stack, two opposite walls of the recuperator chamber being provided with openings, a plurality of non-metallic recuperator tubes communicating at their ends with said openings, an insulated conduit communicating with the openings in one of said walls, said recuperator tubes being composed of non-metallic refractory having a thermal conductivity more than fifty per cent greater than that of re-clay, each tube being open at one end to the atmosphere and opening at the opposite end into said insulated conduit for conducting preheated air to said furnace, powdered coal burners projecting into the furnace at the end of the furnace opposite to its stack connection, a coal pulverizer adjacent said burners, a short by-pass connection from .said insulated conduit to said pulverizer for passing a portion of the preheated air into the pulverizer, and means for projecting powdered coal and preheated air into said burners, the preheated air for the burners being drawn by said means from said insulated conduit.

3. The fuel economizer described in claim 1 in which the air inlet end of each of the recuperator tubes is supported by means of a plurality of tiles of poorly conducting material, the edges of the tiles adjacent the ends of the tubes being undercut and calked with plastic cement to compensate for differences in expansion between the tubes and the tiles and to reduce the leakage between the recuperator tubes and the supporting tiles.

4. The fuel economizer described in claim 1 in which the non-metallic heat exchange tubes are composed mainly of silicon carbide.

WILLIAM H. FITCH. 

